In general, phenol is manufactured by oxidizing an alkyl substituted aromatic compound, such as cumene, to form the hydroperoxide derivative thereof, followed by cleavage of the hydroperoxide with a mineral acid such as sulfuric acid to form a cumene hydroperoxide cleavage mass. The cleavage mass generally contains species such as phenol, acetone, α-methyl styrene (AMS), cumene, cumyl phenol (CP), dimethylbenzyl alcohol (DMBA), acetophenone (AP), AMS dimers (AMSd), tars and heavies, and mineral acid such as sulfuric acid. Prior to separating out the different species and recovering acetone and phenol, the cleavage mass is neutralized with a caustic, such as sodium hydroxide, to prevent the acidic cleavage mass from corroding downstream equipment. Much of the salt is separated and removed from the process in a wash/phase separation step prior to feeding the partially or wholly neutralized cleavage mass to a splitter and further purification columns. However, a significant quantity of salt remains in the cleavage mass entering the splitter, and this quantity of salt becomes more concentrated as the stream passes from one purification column to the next.
In the course of making phenol, the fully or partially neutralized cleavage mass passes through several distillation and purification columns to ultimately form a stream of heavy by-products. The heavy by-product stream may be subject to cracking, and the bottoms of the cracker are usually incinerated. The heavy hydrocarbon by-product stream feeding a cracker or furnace, however, contains a high concentration of the salts of neutralization, typically sodium sulfate. The salts remaining after the wash/phase separation step are carried into the splitter, which separates out ketone as an overhead from phenol as a phenol bottoms stream, into the phenol bottoms stream and into further downstream equipment through the bottoms stream of each purification column, all the way to the cracker or furnace. It is in the cracker and furnace, and in the reboiler for the cracker, where the salts of neutralization settle and are no longer carried through. The settling of the salts in the cracker, reboiler, and furnaces causes operating problems, requiring intermittent shut down to clean the equipment or replace parts. The salts also degrade the value of a tarry mass as fuel for burning. Therefore, it is highly desirable to remove as much salt as possible prior to feeding a cracker or furnace.
Many methods have been proposed for removing salts of neutralization in the manufacture of phenol. One such method, disclosed in U.S. Pat. No. 4,328,377, involves feeding a neutralized cleavage mass to a multi-tray (20 or more) splitter, separating out the ketone as an overhead from a bottom fraction comprised of a crude phenol stream, and recovering the phenol, wherein a liquid layer located near or below the cleavage mass feed site but above the bottoms is withdrawn from the splitter column, phase separating the liquid layer into an oil layer and an aqueous layer containing the salts of neutralization, and recycling the oil layer back to the splitter column at a site below the liquid layer withdrawal site. To enhance the ability of the liquid layer to phase separate, a hydrocarbon having a lower boiling point than phenol and a specific gravity difference of at least 0.03 is added to the liquid layer.
A disadvantage of this process is that large quantities of liquid layer must be removed and processed to sufficiently reduce the amount of salt to the desired level. For example, for every 100 parts by weight per hour of cleavage mass fed to the mutli-tray splitter, 127 parts by weight per hour of a liquid sidedraw was processed in a phase separator. Treating such large amount of liquid requires adding and processing corresponding large quantities of water, and adding large quantities of lower specific gravity hydrocarbon, processing large amounts of oil layer, and increasing the volume of the phase separation vessel. It would be desirable to discharge the salts of neutralization from the process by feeding a phase separator with a small amount of hydrocarbon, discharging low amounts of water containing the salts of neutralization from the process while effectively removing at least 80% of the salts of neutralization from the process, using low amounts of lower density hydrocarbon to enhance phase separation, concentrating the salt in a purge stream discharged from a phase separator to high levels, and/or employing a smaller phase separation vessel. It would also be desirable to employ a process for manufacturing phenol where the amount of water or lower density hydrocarbon lost from the process is minimized or eliminated.